Ero Copper's January 2019 technical report on the NX Gold Mine in Mato Grosso, Brazil describes a 2012 plant running 12 g/t gold ore through three-stage crushing, a ball mill, Falcon and Gekko gravity circuits, flotation and CIL, with overall recovery of 91% to 92%.
The NX Gold Mine is an underground gold and silver mine at Nova Xavantina in Mato Grosso, Brazil, owned at the time of the report by Ero Copper Corp. The Mineral Resource and Mineral Reserve Estimate technical report is dated January 21, 2019, with an effective date of August 31, 2018. The Nova Xavantina Plant has been in operation since 2012. It has a conventional three-stage crush, ball milling, Falcon centrifugal gravity concentration, intensive cyanidation in a Gekko reactor, hydrocyclones, flotation, pre-lime and carbon-in-leach (CIL) of the flotation concentrate, atmospheric Zadra stripping, acid washing and smelting. Feed averages 12.0 g/t gold and 22.0 g/t silver. The ore contains about 8% carbon, which historically caused significant gold losses through preg-robbing.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Crushing rate | 80 | t/h | Utilization 34.2% |
| Grinding rate | 44.5 | t/h | Utilization 39.7% |
| Current throughput | About 120,000 | t/y | Capacity above 300,000 t/y |
| Overall plant recovery | 91 to 92 | % | Over 75% by gravity and ILR |
| Ball mill | 12 ft x 19 ft | Not applicable | 1,400 hp, 400% circulating load |
| Falcon concentrate grade | About 4,000 | g/t Au | Feed about 12 g/t |
| Flotation mass pull | About 2 | % | Gold recovery about 90% |
| Flotation feed / concentrate | 2.5 / 50 | g/t Au | Tailings about 0.3 g/t |
| CIL tanks | 6 | tanks | 12 m3 each, 60 hours |
| CIL recovery | 81 | % | 55 g/t in, about 10 g/t out |
| Loaded carbon | 1,200 | g/t | Not applicable |
| Desorption column | 2 | t carbon | Three batches a week |
| CIL tailings pond | About 450,000 | t | More than 10 years |
| Fresh water / power, 2018 | 5,335 m3 / 13,000 MWh | per month | Averages |
Overview
Ore is crushed to minus 10 mm, ground in a closed-circuit ball mill, and about a third of cyclone underflow is treated in a Falcon centrifugal concentrator. Gravity concentrate is leached in a Gekko intensive reactor. Cyclone overflow goes to flotation, and the small mass of concentrate is leached in a CIL circuit. Loaded carbon is stripped and the gold electrowon and smelted into bullion averaging 40% gold and 25% silver by mass.
Key Process Stages
Crushing
Trucks bring run-of-mine ore from the underground mine to surface buffer piles holding 3,000 t, and a loader feeds a hopper. An apron feeder sends 100 to 500 mm material to a Simplex SXBM 9060 jaw crusher with a 100 mm closed side setting. Undersize goes to a double-deck classification screen, and oversize fractions pass to a secondary cone crusher at 20 mm and a tertiary cone crusher at 12 mm, which close the circuit on the first screening belt. Minus 10 mm material feeds the ball mill. Two operators per shift run the crushing unit.
Grinding and gravity
The 12 ft by 19 ft ball mill has a 30% charge of steel balls up to 80 mm and runs with a 15 inch hydrocyclone. Underflow passes a 2 mm screen with oversize returning to the mill. The Falcon concentrator operates at 60 G in cycles of 19 minutes, and a Gekko intensive cyanidation reactor processes 1,500 kg of gravity concentrate per batch, about 12 hours of operation. Hydrocyclone overflow targets a P80 of 150 µm and passes a 1 mm vibrating screen into flotation.
Flotation
Three rougher cells, three scavenger cells and one cleaner circuit use FLSmidth tank cells of 9 m3 each at about 30% solids. Reagents are 65 g/t of potassium amyl xanthate collector and 30 g/t of Flomin frother. The concentrate moves to CIL, and tailings go to the flotation pond.
Carbon in leach
Six agitated tanks of 12 m3 form the CIL circuit. The first is used only for a pre-lime step with no carbon, and the other five hold carbon at a target 20% by volume. Cyanide approaches 1,200 ppm in the first tank and 300 ppm in the last, and sodium hydroxide controls pH. Slurry passes a small screen to retain fine carbon and goes to the CIL tailings pond. No additional detoxification step is normally used, since cyanide degrades under UV light in the pond, and hydrogen peroxide destroys cyanide when pond water is reused.
Desorption and refining
Zadra stripping at atmospheric pressure uses 0.2% NaCN and 2% NaOH for 30 hours at 95 °C, followed by acid washing at room temperature with 3% HCl, with a first and a final wash. Two Gekko electrowinning cells handle intensive leach solution and stripping solution separately. Cathodes are smelted in 25 L crucibles with borax, potassium nitrate, sodium carbonate and sand as fluxes.
Additional Interesting Data and Summary
The plant staff is 30 direct employees plus 27 maintenance personnel, and it works three shifts a day from Monday to Saturday. It is running well below capacity because of low mine feed. The flotation tailings pond has about 10 months of storage, and thickened inert tailings are removed routinely and used to reclaim artisanal open pits within the NX Gold concessions. The CIL pond is double lined with HDPE.
Process water is pumped from the underground mine, and the tailings circuit is essentially a closed loop, losing water only through evaporation and tailings moisture. The authors found no material risks to the supply of water, power or reagents.
Key Processes
- Jaw crushing and two stages of cone crushing
- Ball milling with hydrocyclones
- Falcon gravity concentration and Gekko intensive cyanidation
- Rougher, scavenger and cleaner flotation
- Pre-lime and carbon-in-leach of the concentrate
- Zadra stripping, electrowinning and smelting
Source: Mineral Resource and Mineral Reserve Estimate of the NX Gold Mine, Nova Xavantina, Mato Grosso, Brazil, Form 43-101F1 Technical Report, January 21, 2019.
